| Hydrogen is one of the harmful elements in the aluminum alloy melts, and it can easily cause casting defects, such as gas porosity, looseness and so on. Therefore, fast measurement of hydrogen content has been an important research subject in the field of aluminum alloy production. However, due to the lack of fast dynamic measurement method, the hydrogen content is mainly measured by the static diffusion method. In this study, a new method of dynamically measuring hydrogen partial pressure of the aluminum alloy melt has been proposed. According to this new method, a system has been designed to fast measure the hydrogen content in aluminum alloy melt. This research mainly includes the absorbing hydrogen characteristics and the new dynamic measurement method, the models of dynamically measuring hydrogen partial pressure and calculating hydrogen content, and the development of new apparatus for fast measuring hydrogen content. The investigation results have proved that the system based on hydrogen partial pressure dynamic measurement can fast measure hydrogen content accurately of the molten alloy in front of furnace. Therefor, it can be used to study the hydrogen diffuse true in the melt, which provides a means of studying the degassing technology of aluminum alloy.First of all, on the basis of further studying the origin of hydrogen, the dissolution way, the existing form, the existing position and the thermodynamics and dynamics of absorbing hydrogen, the relationship between the hydrogen content in the aluminum alloy melt and the hydrogen partial pressure of the melt surface has been analyzed and it does exist. On the basis of deeply researching the relationship between the hydrogen partial pressure and the pressure in the vacuum chamber, a new method of dynamically measuring the hydrogen partial pressure is put forward, and the hydrogen partial pressure can be fast measured by way of dynamically approaching the balance. The theoretical evaluating basis is the pressure change generated by self-modulation chamber. The diffusing direction of the hydrogen can be got instantaneously by monitoring the different pressure between theoretical and real-time pressure in the chamber. When the sign of the different pressure reverses, the diffusing direction of the hydrogen has also changed, and this pressure in the chamber can be regarded as the hydrogen partial pressure in the aluminum alloy melt.Secondly, the relationship between the hydrogen partial pressure and the hydrogen content in the aluminum alloy melt has been established. When the hydrogen atom directly dissolved into the alloy melt, the solubility expression of hydrogen can be achieved by taking the equilibrium constants into van’t Hoff isotherm equation. Through the weighted average of the standard molar free energy of hydrogen in each alloy component, the mol fusion heat formula of hydrogen for general unideal solution can be obtained, and after changing the form of the equation, the corresponding relation formula between the hydrogen content and the melt temperature, hydrogen partial pressure, alloy composition constant A and B can be got.Thirdly, the apparatus of dynamically measuring the hydrogen partial pressure and fast measuring hydrogen content in aluminum alloy melt is constructed. It is composed of vacuum changing volume unit, variable volume drive unit and measurement and control unit and so on. The vacuum changing volume unit consists of vacuum pump, hydrogen can, variable vacuum chamber, vacuum valve group, thermostat bath, isolation unit, and the probe and so on. The variable volume drive unit consists of the motor, decelerating mechanism, limit switch and control circuit. The measurement and control unit consists of pressure difference sensor, thermocouple, data acquisition module and computer. The variable vacuum chamber can be altered by the variable volume drive unit and, therefore, its internal pressure can be continuously changed at uniform speed. The quiet motor and decelerating mechanism are used to change the volume of the chamber, which reduces the influence of the variable volume drive unit on the apparatus and achieves steady changing speed of the pressure. The vacuum pump is used to vacuum the vacuum chamber to the pre-determined value. The pressure difference sensor and the thermocouple are used to monitor the real-time pressure of the vacuum chamber and temperature of the melt. The thermostatic bath can ensure that the gas in the vacuum chamber is changed at constant temperature and, therefore, the influence of temperature on hydrogen diffusion can be reduced. The isolation unit is used to prevent the aluminum alloy melt from the vacuum chamber when the probe is damaged. Data acquisition module transmits the temperature and pressure signals to computer through the serial protocol and computer carries out the calculation and evaluation. The functions of computer software mainly include the action control of the motor, the vacuum pump and the vacuum valve group, the signal acquisition and storage of position sensor, micro-pressure sensor, AD590 and thermocouple, visible operation of computer, data-base manage and malfunction.Forthly, study on a heat-resistant probe with low cost, the probe with special materials by high pressure and high temperature treatment has strong thermal stability, high mechanical strength and appropriate permeability. Therefore, it can be used for fast measuring hydrogen content based on dynamic measurement of hydrogen partial pressure in aluminum alloy melt. And compared with the probe of import instrumentation, it has similar usability, but the cost can be reduced by 90%.Lastly, experiments on fast measuring the hydrogen content in the aluminum alloy are performed by self-developed apparatus, and the experiments are divided into two parts, namely the accuracy testing and the changing rule of the hydrogen content. In the accuracy testing experiments, the system measuring results are contrasted with that of density testing method, hydrogen partial pressure testing method and standard testing method, respectively. In the changing rule experiments of the hydrogen content, the influences of holding-temperature time, degassing crafts, modification treatment on the hydrogen content are studied respectively. Moreover, the relation between the pinhole density and the hydrogen content are also studied. Compared with the density testing method, the absolute error is less than 0.056ml/100g, and relative error is less than 5.99%. Therefore, the apparatus can be used in the practical production. |
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